The present invention relates to an electrostatic sorting apparatus. More particularly, the invention relates to an improved electrostatic sorting apparatus which sorts and refines different constituents in refuse utilizing the difference in electric properties of the constituents and the electrostatic (Coulomb) force induced in the various constituents.
Recently, certain constituents in municipal refuse have been reused as compost. For that purpose, from the raw municipal refuse are removed metals, glass, materials such as ceramics and pebbles, gravel, and plastics while the residual raw refuse is conveyed to a solid waste treatment plant, there finely pulverized are then subsequently conveyed to a composting plant for composting. In the composting plant, there are performed steps of preliminary fermentation and postmaturation fermentation to convert the raw refuse into completely matured compost followed by successive refinements such as vibration screening thereby to obtain a compost product. However, the compost produced through the postmaturation step contains a mixture of finely granulated pure compost solidified in lumps resulting from insufficient putrefaction and decomposition of the organic ingredients of the raw refuse, other ingredients resulting from insufficient putrefaction and decomposition, and foreign materials such glass, plastics, ceramics and pebbles and the like mixed with the refuse which were not successfully removed earlier. Of the mixture, large lumps of the ingredients resulting from insufficient putrefaction and decomposition and foreign materials of relatively large size can be classified and sorted through a vibration screen. However, finely pulverized foreign materials having a size on the order of several millimeters similar to the pure compost pass the screen mesh with the compost mixture. The foreign materials thus mixed in the compost through the classifying and sorting steps are present normally in a concentration of about several percent. However, this is not fully acceptable for agricultural use, and hence such contaminating materials should be removed as far as possible.
The principle of an electrostatic sorting operation is well known and a variety of modified electrostatic sorting apparatuses have been proposed and are in practical use in several industrial fields. A schematic view illustrating the principle of a representative electrostatic sorting plant used in refining ore or food powder is illustrated in FIG. 1. This electrostatic sorting plant has a rotary drum 1 forming a ground electrode, a corona discharge electrode 2 disposed at the side of the drum 1 to which a high voltage is applied, a feeder 3 with a supply hopper disposed above the drum 1 for supplying a mixture M to be sorted into the drum 1, and collecting hoppers 4 and 5 located under the drum 1. Reference numeral 6 indicater a scraper such as a brush in sliding contact with the periperal surface of the drum 1.
The principle of operation of such as electrostatic sorting plant is already well known, and hence only a brief description will be given here. In case that the mixture to be sorted is the forementioned compost, the mixture M includes insulating particles N, designated by small white circles, such as glass and plastics, and electroconductive particles O, designated by solid small black circles, of conductive compost containing a relatively high water content. A strong electric field is applied in the space between the corona discharge electrode 2 and the drum 1 with a corona current flowing toward the drum 1. When the compost containing foreign materials supplied onto the drum 1 from the feeder 3 reaches an area where there is a strong electric field, it is exposed to an ion shower. Due to the ion shower, the conductive particles O of the pure compost are drawn toward the drum 1 causing an ion discharge. Then, the conductive particles O are charged with the same polarity as the drum electrode by electrostatic induction. The conductive particles O of the pure compost are repelled from the cylindrical drum 1 due to the electrostatic force therebetween and resultantly fall forwardly toward the compost collecting hopper 5. On the other hand, the foreign materials such glass and plastics, the insulating particles N, are charged with the same polarity as the corona discharge electrode 2 and accordingly are attracted onto the surface of the drum 1 and rotatably move together with the drum 1. The foreign materials are thus moved to the lowermost position of the drum and accordingly drop into the foreign material collecting hopper 4 due to the force of gravity as designated by an arrow B. Fine foreign materials which remain adhered to the surface of the drum 1 due to their light weight are forcibly scraped off from the drum 1 by the scraper 6.
The conventional electrostatic sorting plant thus described exhibits the following drawbacks in practical use:
(1) Small treatment capacity
As mentioned above, it is necessary, in order to attract the foreign materials onto and remove them from the drum, to charge the foreign materials with ions using a corona discharge. For that purpose, it is necessary to form the foreign materials in a laminar state on the drum to expose them to the corona discharge electrode by limiting the supplying rate of the compost from the feeder. It is necessary to eliminate the superimposition of the foreign materials and the compost on the drum by sufficiently limiting the supplying rate from the feeder so as to evenly distribute the foreign materials on the drum. This results, in the conventional electrostatic sorting plant, in a small sorting capacity per unit length of the rotary drum in the axial direction making it necessary to construct a large plant to achieve a practical processing rate.
(2) Limitations in size and shape of foreign materials to be sorted
As shown in FIG. 2, insulating foreign materials N are moved while adhering to the surface of the drum 1. As to the plastics or film-like chips which are thin and light in weight, however, the distance d from the surface of the chips having a positive charge to the surface of the negatively charged drum is short when the chips are attracted to the surface of the drum so that the resultant electrostatic force is largely exerted between the chips and the surface of the drum and the force of gravity is insufficient to remove them from the surface of the drum even at the lower end of the drum. However, relatively thick ships such as glass have a relatively long isolating distance d therebetween so that the resultant electrostatic force is small. Due to their relatively high weight, they cannot accordingly be attracted onto the drum but immediately slidably drop from the drum into the hopper on the compost side to be thus mixed again with the compost.